1. Field of Application
The present invention relates to an exposure control apparatus, for exposure control of an electronic camera which captures successive images expressing an object such as a preceding vehicle, which is located ahead of a vehicle in which the camera is installed.
2. Description of Related Art
In recent years, vehicle-installed electronic cameras (in general, digital video cameras, referred to in the following simply as cameras) have come into use for capturing images of a region located ahead of the vehicle, with technology having been developed whereby the captured images (that is sets of digital data expressing respective captured images) are processed for detection of objects such as a preceding vehicle. The processing results can be used to generate warning indications to a vehicle driver, control driving of the vehicle, etc. A vehicle having such a camera and processing apparatus installed therein, which are being described, is referred to in the following as the “local vehicle”.
With such technology, it is important that the exposure of the camera be appropriately controlled in accordance with variations in the brightness of the scene captured by the camera, in order to maximize the reliability of recognizing an object such as a preceding vehicle which may appear in an image obtained from the camera.
As described for example in Japanese patent first publication No. 6-253208 (designated as reference document 1 herein), a method of using camera images for recognition of white lines on the road surface has been proposed whereby two laterally extending sections are selected within each image. A first one of these sections is positioned to contain a part of the (imaged) road surface that is currently close to the local vehicle. The data obtained from the first section, in each of successive captured images, are utilized for recognition of white lines on the road surface. The second section is positioned to contain a part of the road surface that is farther ahead of the local vehicle (i.e., is in an upper part of each captured image). Hence the second section contains a region which will be subjected to recognition processing at a future time point, determined by the speed at which the local vehicle is travelling. Designating the average brightness levels of the first and second sections as b0 and b1 respectively, the difference between these is obtained for each of successive captured images. If the difference is found to exceed a predetermined threshold value, then the cameran exposure which will be applied in capturing the next image is adjusted based on the brightness value b1 (i.e., by changing the camera shutter speed, etc).
With the above method of reference document 1, if for example the road surface ahead of the vehicle changes between a brightly sunlit condition and a shade condition, the cameran exposure can be appropriately controlled for each of successive captured images, i.e., such as to prevent the abrupt change in scene brightness from affecting the reliability of white line detection.
However in an actual road environment, the brightness of the road surface will not generally change between a sunlit condition and a shade condition with the change extending uniformly across the road surface in a simple manner. Instead, the changes can take various forms. For that reason, it is difficult to reliably control the cameran exposure by such a method under actual operating conditions.
It has also been proposed, for example in Japanese patent first publication No. 2005-148308 (designated as reference document 2 herein) to use an exposure control apparatus whereby the brightness of the road surface ahead of a local vehicle is measured for use in cameran exposure control, while excluding the effects of white lines (traffic lane markers) formed on the road surface. A video camera on the vehicle obtains successive captured images of a region directly ahead of the vehicle, which contains these white lines. A plurality of areas within each captured image are selectively examined to measure their respective brightness levels, with these areas being predetermined as corresponding to areas of the road surface that are normally outside the white lines when the vehicle is travelling along the center of a traffic lane. The respective brightness values of these areas are measured, and the exposure of the vehicle-mounted camera is controlled based on the results.
On the case of capturing images for use in recognition of a target object (i.e., a 3-dimensional object) such as a preceding vehicle, it would be possible to perform exposure control of the camera based on measuring the brightness of the road surface as described above, since normally that brightness is relatively stable. However the brightness values of various vehicles may differ substantially, so that in the prior art, such exposure control has been performed based upon measuring the brightness of the preceding vehicle which is to be detected. However while such exposure control is being performed, sudden changes in the level of measured brightness may occur, since the preceding vehicle can arbitrarily enter or leave the field of view of the camera, or another vehicle (e.g., having a different level of brightness) may suddenly enter the field of view of the camera, by cutting-in ahead of the local vehicle.
In the prior art, it has not been possible to control the cameran exposure to respond sufficiently quickly to such abrupt changes in the level of brightness that is being measured, so that stable and accurate control of the cameran exposure has been difficult to achieve.